The present invention relates generally to combustible gas sensors, and more specifically, to a gas sensor in combination with a temperature detector.
Improved methods for monitoring catalysts are desired an automotive vehicles to provide accurate hydrocarbon measurements to the engine controller. Because of the high temperatures associated with catalysts the technologies available for such applications are severely limited.
Wide band gap semiconductor silicon carbide (SiC) offers advantages over conventional semiconductors such as silicon and galium arsenide for high temperature, harsh environment and high speed applications. Advances in SiC single crystal growth technology, resulting in increased wafer size of higher quality material with reduced costs has provided an area of particular interest in the development of gas sensors.
Two types of gas sensors developed using SiC technology include metal oxide semiconductors (MOS) and metal insulator semiconductors (MIS). Generally, the MOS gas sensor consists of a semiconductor substrate with an ohmic contact on one side and with the other side covered by a SiO2 insulating layer with a metal gate on top. The metal gate is composed of a metal capable of catalyzing the oxidation of combustible gasses. As a result of the catalytic redox reaction on the gate surface, certain atomic and molecular species are generated which can diffuse through the porous gate to the metal gate/insulator interface where they can ionize. These ions can penetrate through the insulator thereby changing the potential distribution across the device. This changes the potential of the insulator/gate interface and thus the depletion layer inside the semiconductor which in turn shifts the voltage dependent AC admittance characteristic of the device shift along the voltage axis. For a MIS device, the DC resistance characteristics shifts along the voltage axis.
MOS and MIS sensors have an output dependent on temperature of catalytic gate. Therefore, it would be desirable to simultaneously measure temperature and the gas concentration to more accurately determine gas concentration.
The present invention combines a gas sensing structure and a temperature detecting structure within the same sensor. In one aspect of the present invention, a sensor includes a substrate and a catalytic gate electrode disposed on the substrate. The catalytic gate electrode has a first end having a first contact pad and a second end having a second contact pad. A meander is placed between the first contact and the second contact. A third contact pad is coupled to the underside of the substrate. The temperature is measured between the first contact pad and second contact pad while the gas concentration is sensed between the gate electrode and the third contact.
In a further aspect of the invention, a second meander may be placed adjacent to the first meander. The second meander may be used as a heater to heat the device. A heater may be desirable during a cold startup of the vehicle.
In yet another aspect of the invention, a method of sensing gas concentration and temperature comprises the steps of:
measuring a resistance of a meander on a substrate;
determining a temperature associated with said resistance;
measuring an electrical characteristic between the meander and a third contact on a back side of a substrate; and
determining a gas concentration in response to said electrical characteristic and said temperature.
One advantage of the invention is that because the temperature of the catalytic gate is known, the output of the sensor and gas sensor may more accurately be determined. The colocation of the temperature sensor and gas sensor allows very accurate readings of the temperature at the sensor.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.